The present invention relates to the spacing of bundled subconductors of a transmission line, and more particularly, to the control of wake-induced oscillations in the subconductors.
The conductors in transmission lines are subjected to different forms of oscillations which result from different conditions. For example, wake-induced oscillations in a subconductor are substantially different, both in nature and causes than other forms of vibrations, such as aeolian or vertical vibration, and thus require different arrangements for their suppression.
The aforementioned phenomena of wake-induced oscillations in bundled subconductors in transmission lines is a form of wind-induced flutter instability. For example, when wind blows across the bundle, the aerodynamic energy input to the leeward subconductor of the bundle disposed in the wake of the windward subconductor, causes instability and a consequent periodic motion of the leeward subconductor. The motion of the leeward subconductor is self-excited and comes about as the result of the variations in the drag and lift forces in the wake acting of the leeward subconductor. The motion of the leeward subconductor is transmitted to the windward subconductor through the mechanical coupling provided by spacer members connecting the subconductors together in the bundle, and thus, causes it to undergo similar periodic motion.
Several different modes of wake-induced instability can occur. These modes can be generally classified in two categories which are (1) the rigid body modes, and (2) the subspan modes. The rigid body modes are those where the bundle moves as a whole with no relative displacement amongst the points contained within a transverse cross-section of the bundle. On the other hand, the subspan modes are those where relative displacement occurs between the subconductors of the bundle contained in any vertical cross-section. It is known that several different rigid body modes, subspan modes, or combinations of these modes can actually occur in a span of bundled subconductors.
Wake-induced oscillation is relatively rare, but when this phenomena does occur and is allowed to go unchecked, transmission line failures can result. These failures usually occur in the support hardware and/or the spacer members connecting the subconductors, and in certain extreme cases can render the transmission line inoperative. As the subspan modes of the wake-induced instability condition are considered to cause a majority of these failures, the present invention is directed to a solution of the problems involved in the control and suppression of the oscillations resulting from the subspan modes to provide greater reliability in high-voltage power transmission lines.